I have accumulated several SPL/Spectrum Analyzer apps over the years and today I did a quick comparison between them to show how accurate they really are in the bass range, since that is where I tend to do most of my tweaking. It is well known that the iPhone automatically rolls off frequencies below around 150Hz, in order to minimize wind noise being picked-up by the mic while making a phone call.

The apps are as follows:

SPLnFFT

RTA (full/paid version)

JL Audio Tools

Decibel Ultra

The reference curve ("actual") is taken with a RadioShack digital SPL meter with the reading having been corrected through the use of correction tables available on the internet. Tones were played individually from a CD. I'm sure it is not perfect, but the readings should be within 2 or 3 dB of the actuals.

Results show that most apps are not reliable whatsoever below 160Hz (as expected). The absolute readings are also all over the place, but for this purpose they don't really matter. It is the accuracy of the relative readings that is important. Above 160Hz, Decibel Ultra and SPLnFFT are fairly accurate, but below 160Hz only SPLnFFT is accurate and only down to about 63Hz. Below that, it's a crapshoot!

For the record, the iPhone was held in head position with the mic pointing straight up.

I have that, too. I never got it to work. Paid $60 for TrueRTA, $50 for the MicMate, and $65 for the mic. Paperweights. Turns out you actually need a laptop with a super duper sound card, like a $2000 Alienware or something like that, otherwise the readings are off. You can also get an external USB soundcard (about $100), but then you have so much shit hanging off the laptop with all these wires, and need to get a mic stand for the mic, that it is just not convenient to use. Plus the software is pretty complicated to use and you need to calibrate with the sound card to get it to work properly. Call me crazy but that's just not worth it. If you want a deal on a mic and micmate, let me know!

Interestingly, StudioSix just came out with a cool mic that attaches directly to the iPhone sync connector, and it works with their RTA softare. Pretty convenient, and pretty accurate (so they say), but at $199 it is not exactly a bargain.

My test shows though that you can use the $4 SPLnFFT app to do some pretty accurate level setting, and that was the purpose of this thread. This is the only app (out of the ones I have) that's going to measure the level of the underseats accurately.